When a body of aluminum is melted in a furnace for purposes of casting or the like, dross forms on the surface of the molten aluminum and must be periodically removed, for example by skimming or similar operation. The removed dross generally contains substantial amounts of free aluminum as well as aluminum oxides and certain other metals and metal salts, such as magnesium, manganese, and lithium depending on the nature of the aluminum or aluminum alloy being treated.
U.S. Pat. No. 4,877,448 discloses a process for the recovery of free aluminum and aluminum oxides from aluminum dross and aluminum scrap (hereinafter, for convenience, collectively referred to as "dross" or "aluminum dross") comprising heating the dross in a high temperature furnace using a plasma arc torch fed with air, or other oxidizing gas such as a mixture of air and oxygen or air and nitrogen as the arc gas. The use of air as the arc gas in comparison to nitrogen provides greater heat output with the same electrical input (KW/hr), resulting in a shorter processing time and substantially higher throughputs. Moreover, nitride formation is reduced since the hottest part of plasma which is closest to the torch is oxidizing, not nitriding. The oxide, Al.sub.2 O.sub.3, which is formed is stable and not an environmental problem. The process is more economical since air is much cheaper than nitrogen or argon, or any other available inert arc gas.
Dross recovery systems of the type using air as the arc gas have conventionally utilized two furnaces, i.e., furnaces 1 and 2, each equipped with a separate plasma torch. However, to avoid excessive installation costs and high electrical energy requirements, the system is designed in order that only one of the torches is lit off or operated at any given time. Thus, in operation one of the two furnaces is charged with dross and thereafter the plasma torch lit off, and the dross in furnace 1 undergoes treatment for recovery of free aluminum. During the operation of furnace 1, furnace 2 is charged with dross; and as soon as the cycle time is completed in furnace 1, which requires approximately 4 hours, the plasma torch at furnace 1 is shut down and the plasma torch at furnace 2 is lit off. The dross in furnace 2 is then processed, again requiring approximately a 4 hour cycle time. During the processing of the dross in Furnace 2, the free aluminum and the non-metallic by-products (NMP) are removed from furnace 1, with furnace 1 then being again charged with dross for a further aluminum recovery cycle once the dross in furnace 2 is completely processed.
In the conventional process as above described, in order to maintain the internal furnace temperature and the temperature of the dross being treated in the desired temperature range, the plasma torch is initially fired followed by a sequence of shutting the torch down and relighting the torch. Thus, the need to have alternative firing followed by a period of shutdown is a result of the plasma from the torch being hotter at its core, i.e., about 12,000.degree. C., than at its outer surface, i.e., about 1,000.degree. C., with varying temperatures in between the core and outer surface. This hot core of the plasma, although believed desirable since it is believed to enhance the cracking of the dross shell surrounding free aluminum, leads to overheating if a sequence of alternate firing and non-firing of the torch is not used. During the time when the torch is not lit off, it is necessary to flood the furnace with an inert gas such as argon. The use of this procedure substantially lengthens the cycle time, and additionally the cost of argon gas used in the process is relatively expensive.
Accordingly, although the process as defined in U.S. Pat. No. 4,877,448 operated as above described is a highly desirable process, there is need for improvement in the process, particularly in reducing cycle time.